Final answer:
Spica casting for femur fractures with less than 2 cm of shortening is typically used for children under the age of 6. Radiographs help estimate a child's age based on skeletal maturation and growth plate development.
Step-by-step explanation:
Spica casting is often considered for femur fractures in children who are generally under the age of 6. When there is less than 2 cm of shortening in the fracture, this treatment method can be particularly effective due to the ability of young children's bones to heal and remodel quickly. Radiographs, or X-rays, of a child's femur can be quite informative; the presence, size, and degree of ossification of growth plates, as well as the overall size and shape of the bones are indicators of the child's skeletal maturity and, hence, approximate age.